iPhone 4 iOs 6.1 Signal issues

I've had my iPhone 4 for a while and never had a problem, however, when I updated to 6.1 I have signal issues. I am on o2, I have normally 3-5 all the time 3G signal. When I updated to 6.1 I now have no service at least 30% of the time or my signal goes from full signal to 1 bar to no service to full signal within seconds of each other. It makes no difference if I am connected to wifi or not and does it in different locations. Its not my sim because I've used to sim in a different phone so its nothing to do with o2-uk. I've turned my 3G off to see if that makes a difference it doesn't. I'm out of warranty now and when I rang apple support they wanted to charge me to discuss it! Not impressed! Any advice or fixes?